This invention relates generally to a method and apparatus for tightening a series of threaded fasteners so that all the fasteners are under the same tension forces. Specifically, the invention relates, in one embodiment, to a method for tightening the cylinder head bolts in an internal combustion engine to the same degree of tension.
The present invention departs from prior art practices and provides an exceedingly accurate method of tensioning threaded bolts which are used to fasten or attach one element to another using bolts, technically referred to as cap screws. The tensioning system eliminates the margin of error associated with current torque wrenches and torque application systems which are. based on the measurement of xe2x80x9ctorquexe2x80x9d or fastener tensioning through the frictional intermediate interface of the bolt and threads of the fastener receptacle. Further, the present invention allows real time equal starting, running in, and tensioning of fasteners and computer certified automated quality control of the assembly process.
To better understand the significance of the present invention, a brief description of prior art practices follows. When threaded fasteners are tightened, they are actually stretched not unlike a rubber band holding together two blocks of wood. The current method of tensioning fasteners is referred to as applying torque to the fastener. Current torque wrenches measure the force of tension applied to a lever arm at a unit radius distance away from the center axis of the fastener. In the United States the current common measurement is the foot pound. In the current method, inaccuracy is created by the friction between the threaded fastener and the threaded receptacle into which the fastener is driven. Fasteners and the threaded receptacles are most commonly alloys but may also be ceramics, cermets, plastics, or combinations of these. Alloys, which are crystalline structures, and cermet matrices are almost never homogeneous and have highly variable frictional characteristics. Forming threads in these materials not only exposes unknown surfaces, but the tools used to form the threads, further imparts unknown frictional characteristics unique to the tool marks or striae created by each unique tool. Given to the large number of variables in the materials and the screw thread forming process, measurement of fastener tensioning through the frictional interface used by current art torque wrenches will always provide a sizable percentage of error and preclude reliability in regard to definite repeated uniform tensioning of the fastener and the fastened unit.
Angular torque has been adopted virtually universally as the measure of measuring tension strain. Accuracy has increased, but the current angular torquing method still starts with a low initial starting torque value, from which torque angles are later measured and inaccuracies due to friction irregularities have not been eliminated.
Presently used torque wrenches, either hand or machine operated, are frictional compression or tension loaded devices that release when the set threshold setting is reached. These are referred to in the industry as torque limiting devices. Since these devices are based on springs and friction, they are subjected to increased inaccuracy due to repeated use and must be re-calibrated and compared to a reference which is, currently, an analog, frictional spring loaded unit.
The current method of setting up a multiple fastener assembled unit such as an engine block, a gasket and a cylinder head requires tightening individual fasteners in a spiral pattern in steps of lesser that the ultimate desired torque value, either measured by a torque wrench or an angular torque device. This method requires that the components be over designed in strength to prevent displacement or warping during the assembly process.
The current method of quality control is to have an operator place a paint mark of each fastener that he or she has securely tightened. Thus, another object of the present invention is to provide a method whereby all bolts can be tightened simultaneously and uniformity to the same tension.
Previous attempts to solve the problem of uniform tensioning of threaded bolts have resulted in inventions that have addressed methods of tightening threaded fasteners (bolts or cap screws) with quantitative measurement of tightness and the certification of tightened fasteners. None provide for the indexing of the actual referenced starting point of a threaded fastener into the threaded unit, the digital measurement of fastener elongation within the threaded recess and the integrated xe2x80x9cquality controlxe2x80x9d certification of the assembled unit. For example, U.S. Pat. Nos. 3,970,155; 4,091,664; 4,328,709; 4,958,541; 5,172,616; 5,890;406; 5,911,801; and 6,021,555 are all illustrative of prior art. On the other hand, U.S. Pat. No. 6,021,555 granted to Applicant is helpful in appreciating the current invention; however, the rest of the cited art may be suitable for a particular purpose that each addresses but none would be suitable for the intended purpose of the present invention as herein described. Detailed discussion of these prior art patents is set forth below:
This patent discloses conventional beam displacement torque wrench with an attachment that allows presetting a torque value, wherein the attachment provides an audio and a visual signal to the operator, when the preset torque is reached. The beam displacement torque wrench is well known in the prior art and measures torque as a factor of the bending of a wrench beam in relation to a reference indicator to which no bending force is applied.
In this patent an analog torque limiting wrench is described which registers torque as pressure imparted upon a piezoelectric strain gauge. When a preset threshold level of strain imparted by the wrench is reached, a signal is sent to a vibration creating electric motor that provides the user with a tactile signal.
This patent describes a device that simultaneously measures both xe2x80x9ctorque anglexe2x80x9d and torque resistance of the threaded fastener as it is tightened into an assembled threaded recess. While this device provides a digital readout of the turning angle of the fastener being turned, the unit""s primary aim is to prevent over tightening of bolts by measuring the relationship between the torque applied and the angular displacement of the fastener.
The device disclosed is a torque wrench with an electro mechanical torque limiting release mechanism. For quality control purposes, when a preset torque is reached on the device, a felt tip marker is used to mark the bolt head to certify that the particular bolt has been correctly tightened.
Described is a version of the well known and well used spring loaded torque limiting wrench, which releases when a preset torque level is reached. This particular device claims increased accuracy over prior release mechanisms on the basis of its unique design and components.
In these prior art devices the parameter being measured is the angular force required to turn a bolt in a threaded recess. Thus, a primary object of my invention is, instead, to measure a bolt""s longitudinal position.
The disclosure of the patent is incorporated herein by reference and it provides for an automated method of starting threaded fasteners into tapped recesses. In this invention the process begins by precisely locating the initial xe2x80x9czeroxe2x80x9d entry point of thread engagement. Having located the initial referenced xe2x80x9czeroxe2x80x9d starting point, the number of turns or a purely angular torque determines the position of the bolt. The error prone frictional pre-load initial torque value is completely eliminated.
This invention represents the electronic duplication of the current angular process that depends on an initial pre-loaded torque, applied in a similar manner with a conventional torque wrench. Recognition of a xe2x80x9czeroxe2x80x9d starting point is not disclosed.
My invention and the improvements it introduces will be more fully understood from the summary of the invention and detailed description which follow.
In one aspect, the present invention involves inserting a threaded fastener into a threaded element, locating the initial zero starting point of the receptacle and fastener threads by rotating the fastener clockwise and, digitally measuring the angular degree of motion imparted upon the moving fastener to provide assured definite elongation of the fastener into the tapped hole thereby providing optimum fastening.
In one aspect, my invention is a Basic Unit for accurately tensioning threaded fasteners comprising a means for locating the initial zero starting point of a threaded unit in a threaded recess, a motor for driving a chucked threaded fastener in a clockwise direction a magnetic notched wheel having evenly divided number of segments within the 360 degrees of a circle, such notched wheel having no permanent magnetic qualities and preferably may be the notched rotor of a stepper motor; an electromagnet axial rotation sensor comprising an iron core and solenoid, such electromagnet""s pole being disposed to pass across a notch of the magnetic notched wheel, and such electromagnet be part of a stepper motor; means for converting the threshold magnetic signal generated as the notch of the wheel passes over the electromagnet and converting such signal to a binary xe2x80x9c1xe2x80x9d signal; a means for counting the digital signal provided to an axial sensor provided for the fastener as it is driven and for converting the signal into a measurement of the driven axial distance of the fastener; and, means for counting the digital signal provided to the axial sensor as a function of the threaded fastener""s stretched length inside the threaded recess after the fastener is fully driven in.
Accordingly, one object of the invention is to totally eliminate the measurement of torque or fastener tightening tension through the unknown and irregular frictional interface as has been the practice of the prior art.
Another object of the invention is to provide a digitally readable system to determine the exact position of the fastener as it is rotated and driven into a threaded receptacle. Thus, axial position determines ultimate tensioned length to fully tighten the fastener.
Another object of this invention is to provide a fully digital method of measuring torque or tension applied to a fastener and to eliminate the inaccuracy introduced by wear to torque driving machine components.
Yet another object of the invention is to provide a method of near perfect repeatability of inserting a fastener made with equal specification tolerances into a receiving fastened unit of equal specification tolerance.
A significant object of the invention is to provide simultaneous, evenly distributed, micro incremental tightening of multiple fasteners to establish a near perfect equally distributed tightening of gasketed components in assembled units. Such units can be made lighter due to minimized distortion of the components to be assembled.
An additional object of the invention is to provide an extremely accurate method of calculating the quantitative xe2x80x9cdesign safety marginxe2x80x9d for assembled units.
Another object of the invention is to provide computer certifiable quality control of the components assembled, without manual human intervention.
The aggregate object of the invention provides for a fully automated system of starting, running in, accurately tightening, and quality control certification of the assembled unit in one continuous operation.
By referring to the drawings and detailed description below it will be understood how my invention accomplishes the foregoing objects.